Chemical analysis or other testing of liquid samples typically requires collection of such samples from containers for subsequent dispensing on chemistry slides or the like for analysis. Pipettes and similar aspirating devices, either manual or automatic in nature, are ordinarily employed for this purpose.
Depending upon the nature of the analysis to be performed, precise volumetric control of the liquid samples may be required. Samples to be subjected to analysis, control substances, and calibrator fluids frequently must be dispensed in a controlled manner, at predetermined volumes and dispensing rates.
A number of factors must be considered in order for accurate chemical analysis to be achieved. Among these are accurate volumetric aspiration of the liquid sample, minimal liquid perfusion, minimal exposure of the sample to air, minimal drop formation about the exterior of the pipette tip, and control of the dispensing rate.
A typical pipette mechanism for this purpose includes a movable piston arranged within an associated cylinder. By submersion of the free end of the cylinder into a fluid sample, vacuum created within the cylinder by relative movement of the piston causes the liquid to flow into the cylinder, under the influence of external barometric pressure. This flow is governed by the volumetric movement of the piston, as well as the vapor pressure of the liquid, the liquid surface tension, and capillary action. The piston and cylinder arrangement is then removed from the liquid so that the sample can be dispensed as desired.
Because sample sizes can be quite small in volume, a significant quantity of the liquid within the pipette cylinder can be lost if a drop is allowed to form on the free end of the device, thereby compromising accuracy. Such drop formation can result from movement of the piston within the cylinder, fluid adhesion to the outside of the device, thermal and vapor pressure changes within the cylinder head volume above the liquid, and changes in the barometric environment.
A reduction in fluid losses can be achieved by preventing a drop from forming once the pipette is removed from the sample container. In particular, drop formation can be prevented by causing a small air pocket to be formed at and within the open end of the pipette by sucking back the liquid sample with a small additional backward movement of the piston. The small air pocket compensates for cylinder head expansion, minimizes exposure of the sample to the atmosphere by reduction in the exposed surface area, and desirably prevents the formation of a drop on the end of the device that can be inadvertently removed through physical contact or the like.
During dispensing of the sample for analysis, it is ordinarily necessary to achieve accurate sample volume control, accurate drop placement, and controlled dispensing rates in order to achieve precision during subsequent analysis. Since some analysis requires several liquids to be sequentially applied to a single slide, the differing viscosity of the liquids, and their chemical composition may require different dispensing rates, or like variations during analysis.
The present invention provides a liquid aspirating and dispensing system which is desirably suited to facilitate efficient, accurate, and automated liquid sample handling for subsequent analysis and the like.